Can a Low Voltage Current Transformer (CT) be used in a Medium or High Voltage application? Surprisingly, it is a question we get a few times a year and thought it would be beneficial to put this out.
Firstly we must understand the configuration of the switchgear. For example, if the switchgear has a system voltage of 12kV and the CT will be installed on the bare busbars in the medium voltage section. Then yes you will need a CT that's fully rated, and generally, that's where the thought & decision process ends.
But let us dig a little deeper. If the CT is to be installed directly on medium voltage busbars, accordingly it must have an insulation level at least equal to that of the system/cubicle. However, if installing a CT on the medium voltage cable (which is already insulated), in this case, the CT can be a low voltage type (e.g. insulation level 0.72kV) as the cable provides the medium voltage insulation. It is this element that is missed by design engineers and as a result, adds unnecessary cost to the switchgear.
Another scenario is a neutral current transformer, typically on outdoor installations where the CT needs to be installed on an MV or HV system but on the neutral circuit. As such, it will not see the higher MV & HV voltages, so it's possible to use a lower insulation rating for the current transformer. It is critical to note on an MV or HV system were the CT will see "line voltage". Then a fully rated insulated current transformer will be required.
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